Now, a seven-year project has been given the go-ahead to improve the sensitivity of the American observatories' instrumentation by a factor of 10.

The two widely separated installations - one in Washington State; the other in Louisiana - are remarkable technological undertakings.

They fire high-powered laser light down long, L-shaped, evacuated tunnels. The two separate beams are then bounced back by mirrors to their starting point, where they are recombined at detectors. If gravitational waves have passed through the beams, the light should show evidence at the detectors of having been ever so slightly disturbed.

But the expected weakness of gravitational waves means only astrophysical phenomena on a truly massive scale, such as exploding giant stars, are likely to generate detectable signals.

And even then, the technology has had to be pushed to the limits to get into the sensitivity ranges demanded - to measure disturbances in the set-up equivalent to one one-thousandth of the width of a proton, one of the particles that make up all atoms.

'Dark' cosmos

The "Advanced Ligo Project", will see the US National Science Foundation fund $205m-worth of improvements through to 2014. Researchers believe the changes will allow detections of waves to become a regular occurrence, perhaps on a daily basis.

Although sited in the US, the Ligo venture is very much an international partnership, with researchers in Europe playing leading roles.

Much of the new technology that will go into the updated Ligo has been trialled on the German-UK observatory known as GEO-600.

This includes the ultra-stable, high-powered lasers that will catch the waves; and the rock-steady mechanisms that hold the "test mass" mirrors at the ends of the tunnels.

The mechanisms used to hang mirrors in GEO-600 will go to Ligo

"The UK and Germany made a big difference in getting Advanced Ligo funded; the fact that both countries were willing to put money into it," explained Professor Jim Hough, from the University of Glasgow, and a member of the international Ligo Scientific Collaboration.

"There is another $30m, which is the UK-German contribution, on top of the $205m."

Regular detections of gravitational waves would usher in a new era in astronomy - one that was not dependent on the observation of light.

This is vital because most of the cosmos is "dark"; the majority of its matter cannot be seen with traditional telescopes.

A major observation campaign is planned for late 2008 when some interim enhancements at Ligo will come online. The US installations will also be joined in their hunt at that time by the French-Italian observatory known as Virgo.

This will allow GEO-600 to engage in a period of research and development work.

"When a discovery is made we will want to see signals in all the observatories. It will give us confidence; we will know it is not some spurious random event," said Professor Hough.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.